The present embodiments relate to wireless communications systems and, more particularly, to interference-limited wireless communication systems with packet retransmission in response to detected packet error.
Wireless communications are prevalent in business, personal, and other applications, and as a result the technology for such communications continues to advance in various areas. One such advancement includes the use of spread spectrum communications, including that of code division multiple access (“CDMA”) and wideband code division multiple access (“WCDMA”) cellular communications. In such communications, a user station (e.g., a hand held cellular phone) communicates with a base station, where typically the base station corresponds to a “cell.” CDMA communications are by way of transmitting symbols from a transmitter to a receiver, and the symbols are modulated using a spreading code which consists of a series of binary pulses. The code runs at a higher rate than the symbol rate and determines the actual transmission bandwidth. Another wireless standard involves time division multiple access (“TDMA”) apparatus, which also communicate symbols and are used by way of example in cellular systems. TDMA communications are transmitted as a group of packets in a time period, where the time period is divided into slots (i.e., packets) so that multiple receivers may each access meaningful information during a different part of that time period. In other words, in a group of TDMA receivers, each receiver is designated a slot in the time period, and that slot repeats for each group of successive packets transmitted to the receiver. Accordingly, each receiver is able to identify the information intended for it by synchronizing to the group of packets and then deciphering the time slot corresponding to the given receiver. Given the preceding, CDMA transmissions are receiver-distinguished in response to codes, while TDMA transmissions are receiver-distinguished in response to orthogonal time slots.
In certain prior art packet communication systems as detailed later, packet retransmission is often requested when a received packet is detected to be in error. This scheme is often referred to as automatic retransmission request (“ARQ”), and ARQ is intended to reduce or eliminate the effects of packet error, as is often desirable in systems where packet reliability is of high importance such as in multimedia applications. Typically, ARQ is achieved by including some type of error check in each transmitted packet, such as a cyclic redundancy code (“CRC”) at the end of a packet and that relates to the data in the packet. When the packet is received, the receiver decodes the packet and, hence, also the CRC code, further determining from the code whether the packet was received without errors. The receiver also has a wireless feedback communication link to the transmitter, and in connection with ARQ there is often an acknowledgment feedback signal along this link such as in the form of one of two complementary signals designated ACK and NACK. If, from the CRC (or alternate code/method), the receiver detects no errors in a given packet, then the receiver returns an ACK signal to the transmitter, whereas if the receiver does detect an error in a given packet, then the receiver returns a NACK signal to the transmitter. In response to the NACK, the transmitter retransmits the packet corresponding to the NACK, that is, that packet for which an error was detected. This retransmission may take place several times for a same packet, where typically there is a limit on the number of retransmissions, after which the packet is deemed unusable by receiver and as a result the packet is discarded.
Another technology that relates in part to ARQ, and that is also used in certain prior art packet communication systems as detailed later, is referred to as hybrid ARQ (“HARQ”), where HARQ differs from ARQ in that HARQ recognizes that error-containing packets may still provide some useful data at the receiver. In other words, recall from above that the receiver discards the entire packet under ARQ if, after repeated retransmissions, an error free packet is not received. In contrast, HARQ systems attempt to extract some or all of the data from packets that have been deemed to include errors. There are various types of HARQ systems known in the art. One example of a HARQ system is known as Chase combining. In a Chase combining system, the receiver performs some coherent combining on multiple received packets that are received as re-transmissions of an originally-transmitted packet. Another example of a HARQ system is known as an incremental redundancy system. In an incremental redundancy system, the data bits in each retransmitted packet are the same; however, different encoding bits are used for each different retransmission. Further, the receiver is made aware of the different encoding schemes and, thus, it attempts to decode each different packet it receives representing a retransmission in view of the encoding bits anticipated to be applied to that packet. The results of each such decode are then combined in an effort to accurately predict the packet data.
Having introduced ARQ and HARQ systems, note that the above introduction also states that such systems are used in certain prior art packet communication systems; in this regard, the present inventors have observed that those systems as studied in the literature have been confined to additive white Gaussian noise (“AWGN”) or typical fading channels. Such systems typically provide a transmitter with a single transmit antenna and a receiver with a single receive antenna and, thus, the channel between them may be static wherein noise is the primary variance, or in the case of the fading channel there may be additional diversity due to the fading characteristics. Thus, heretofore, HARQ has only been implemented in these types of systems.
More recently there have been developments into wireless communications systems that provide additional types of signal diversity and that are often more practical, but these same systems are interference-limited. In other words, due to the transmitter and/or receiver structure, multiple symbols share the same channel between the transmitter and receiver and, hence, interference is introduced between different symbols communicated from the transmitter to the receiver. As a result, the receiver requires functionality to suppress the interference. Such systems are included in many forms. As one example of an interference-limited system, there are high data rate multiple antenna systems such as multiple-input multiple-output (“MIMO”) systems. In MIMO systems, each transmit antenna transmits a distinct and respective data stream; the symbols in each stream therefore interfere with the symbols in the other stream(s), that is, there is spatial interference since different transmit antennas are used to transmit different data streams. As another example of an interference-limited system, there is the above-introduced TDMA systems. In TDMA systems, there are frequency selective channels with long impulse responses; this causes so-called intersymbol interference (“ISI”), and typically equalizers are used to mitigate the ISI. As yet another example of an interference-limited system, there is the above-introduced CDMA systems. In CDMA, one type of multipath interference effect is multiuser interference (“MUI”). Moreover, since CDMA implements orthogonal symbol streams, often the orthogonality reduces ISI to a negligible value and, as a result, often a less complex receiver structure may be implemented in a CDMA system. However, there is often required a lowering of the spreading factor, that is, the number of modulating chips per transmitted symbol; this spreading factor reduction also reduces the benefit of orthogonality and consequently increases the concern for ISI even in CDMA systems. Still other examples of interference-limited systems can be ascertained by one skilled in the art, including any combination such as MIMO TDMA or MIMO CDMA.
Given the preceding, the present inventors further recognize that while HARQ systems have heretofore been described relative to systems without interference, no provision has been made for implementing HARQ into an interference-limited system. In view of the preceding, therefore, the preferred embodiments address the drawback of the limited application of prior art HARQ systems to systems that are not interference limited; further, such prior art systems, while permitting HARQ and thereby obtaining the benefit of increased packet reliability, suffer from a lack of speed or throughput as compared to interference-limited systems. As such, the present embodiments endeavor to provide various preferred embodiments representing a combination of HARQ (or a comparable methodology) with interference-limited systems, as detailed below.